Showing papers in "International Journal of Offshore and Polar Engineering in 1993"

Bending Stiffness In A Simulation Of Undersea Cable Deployment

Abstract: Bending stiffness is introduced in a three-dimensional model for submerged cable dynamics to eliminate singular behavior when cable tension becomes zero. The equations of motion are written in a local tangential-normal reference frame, and are simplified by neglecting the torsional rigidity of the cable. A centered-centered finite difference algorithm is used for the numerical simulation. The addition of bending stiffness eliminates the singularity for zero tension. However, because the bending stiffness is small, sharp gradients in the shear forces and bending moments occur at the boundaries. In this paper two sets of results are presented: configuration of an anchoring system during a steady tow, and the tension and geometry of a cable immediately following touchdown on the seafloor. The first result illustrates the discretization error caused by the sharp gradients in the curvatures and shear forces. The second result provides evidence that the bending stiffness does prevent singular behavior for negative or zero tension. It also illustrates that the contact of the cable with the seafloor only affects the cable geometry close to the seafloor.

Impulse Turbine With Self-Pitch-Controlled Guide Vanes For Wave Power Conversion: Performance Of Mono- Vane Type

Abstract: An impulse turbine with self-pitch-controlled guide vanes was proposed by the authors in a previous paper. The unsteady characteristics of this turbine have been investigated experimentally by the use of turbine test equipment in which the sinusoidally reciprocating flow conditions are simulated. The results have been compared with those of a Wells turbine. Furthermore, in order to clarify the usefulness of quasi-steady analysis of this turbine, they have been also compared with the analytical results calculated on the basis of the experimental data obtained by the model testing of a turbine rotor with fixed guide vanes under steady unidirectional flow conditions. As a result, it has been clarified that the impulse turbine presented here is superior to the Wells turbine in overall characteristics, and the quasi-steady analysis is available for this turbine.

Flashover Performance of Insulators In the Presence of Short Icicles

Abstract: This work chiefly presents the results or'a laboratory investigation of ac leakage current and ac flashover on a short insulator string. The string was partially iced with supercooled droplets at _1.5°C simulating a rime fog, this being one of the factors responsible for a large number of flashovers on Ontario Hydro's 500 kV system in March 1986. It was found that the leakage current of insulators in the presence of short icicles was very small and consequently no flashover occurred under normalline-to-ground voltage applied to the insulators. Flashover was produced only under an applied voltage about 80% higher than normal voltage. It was concluded that the presence of fog was associated with the flashovers occurring in March '86, but that other parameters such as insulator surface contamination and water condensation were also active factors contributing to these flashovers.

Second-Order Wave Radiation Of Three-Dimensional Bodies By Time-Domain Method

Abstract: The nonlinear wave radiation of a surface-piercing body of arbitrary shape in three dimensions is investigated by a timedomain second-order method. In this approach, Taylor series expansions are applied to the body surface boundary condition and the free surface boundary conditions, and Stokes perturbation procedure is then used to establish corresponding boundary value problems at first and second order with time-independent boundaries. These are treated by a time-stepping procedure, in which the wave field at each time step is solved by a boundary integral equation method based on Green's theorem. Numerical results including free surface profiles and hydrodynamic forces are presented for the case of a truncated surface-piercing circular cylinder undergoing specified sinusoidal surge and heave motions. Contributions to the hydrodynamic force from the various second-order force components are highlighted. It is found that steady state solutions are attained over a reasonably short duration of simulation time, and that the method is accurate, computationally efficient, and numerically very stable.

Review Of Hydrodynamic Challenges In Tlp Design

Abstract: From the early developments of tension leg platform (TLP) a number of hydrodynamic challenges have emerged. Considerable research and engineering effort has been spent, but there is still considerable scope for further work in this research area. This paper outlines the various hydrodynamic effects that must be accounted for in TLP design and how they practically relate to the safety and operation of TLPs. A list of suggested priorities for further hydrodynamic research is given.

Local Bending Stresses In Cables

Abstract: After a discussion of the various types of stiffness possible in a suspended cable responding to live loading, theory is presented for the steady-state response of a taut, flat cable in which allowance is made for flexural effects at its terminations. The steady-state response could be taken to refer to strumming effects due to, say, vortex shedding in an underwater application where, if the cable material is synthetic, it will be straight on account of the neutral buoyancy, or nearly such, of synthetic cables. Equally, it could refer to a bridge stay cable. In any event, the model presented for behaviour in the flexural boundary layers of cables provides a useful starting point for the development of fatigue theories for different types of response such as that induced by vortex shedding, or by parametric excitation (where motion at one termination may give rise to lateral motion in the cable). The appendix contains formulae for any type of cable whether solid, parallel lay or helical lay.

Self-Burial Of Pipelines At Span Shoulders

Abstract: The process of self·burial of pipelines at span shoulders has been investigated experimentally in the laboratory in steady currents. Only the noncohesive sediment bed is considered. A rigid cylinder has been used as the pipe model. In all the tests, the experimental conditions were arranged such that the pipe is supported by a sand ridge in the middle. When the length of this supporting ridge is decreased to a critical value, the pipe begins to sink in the sand due to soil failure. The scour, the sinking and, at a later stage, the backfilling processes were monitored by video in plane and side views simultaneously. A simple formula adopted from soil mechanics regarding the bearing capacity of soil was found to give a satisfactory result in relation to the sinking of the pipe at the span shoulder.

Hydrodynamic Forces Acting On A Floating Body In A Harbor Of Arbitrary Geometry

Abstract: A new method for computation of hydrodynamic forces acting on a floating body in a harbour of arbitrary geometry was developed. This method is composed of a finite element method for solving the harbour response and a three dimensional integral equation method for solving the floating body response. These two methods were combined with each other by making use of the eigen function expansion method. The finite element method for the harbour response problem is based on the mild-slope assumption. The integral equation method for the floating body response problem is based on the three dimensional potential theory with the linearized free surface condition. The overall solution is obtained by solving the combined matrix which is obtained by combining the harbour response coefficient matrix with the floating body response coefficient matrix. The accuracy of the method was verified by comparing some of the numerical results with results of previous research. The influence of the heading angle of the floating body and the bottom topography were investigated. Results showed that the influence of the heading angle was significant, however, the influence of the bottom topography was not significant.

Wave group evolution, wave deformation, and breaking: simulations using longtank, a numerical wave tank

Abstract: Non-linear, planar wave trains have been simulated numerically using an efficient computational method. This method utilized matrix diagonalization, based on multi-subdomains. The waves are created by a plunging wedge wavemaker and followed to downtank distances, x/ of O(102). High resolution of wave shapes was obtained. The numerical tank, LONGTANK, was used to study non- linear sideband growth, strong group evolution, wave formation, energy redistribution in the deformed waves, and jet formation (breaking). Comparisons of steepness at breaking and distance to breaking, as they depend on wave steepness, have been made, with excellent agreement. Similar agreement is shown between simulated and photographed wave shapes in the final stages of breaking. A criterion for the onset of breaking is suggested by the simulations.

Wave slamming on a horizontal circular cylinder

Abstract: This paper examines the impact loads due to waves interacting with a horizontal circular cylinder located near the still water level. Numerical models based on the use of a slamming coefficient are indicated for simulating the time history of the vertical force on both a fixed and dynamically responding cylinder. An approach based on an impulse coefficient is proposed for estimating the maximum response of a dynamically responding cylinder. Experiments have been conducted to measure the corresponding slamming and impulse coefficients, as well as the impulse rise-time and duration. Corrections to the measured coefficients account for buoyancy, dynamic response and free surface slope are indicated. The coefficients exhibit a considerable degree of scatter, even when the various corrections are taken into account. However, the degree of scatter of the impulse coefficient is notably less than that of the slamming coefficient. The practical application of the various approaches described is illustrated.

Simulating The Dynamics Of Underwater Vehicles With Low-Tension Tethers

Abstract: A numerical technique for calculating the two-dimensional motions of a tethered underwater vehicle is presented. The method is unique because of its ability to simulate accurately the dynamics of cables that are under low tensions. This is accomplished by incorporating bending stiffness into the cable equations and thus removing the singularity that occurs when the cable tension becomes zero. Numerical results for several tether-vehicle maneuvers are presented.

Tube Collapse Under Combined Pressure, Tension And Bending Loads

Abstract: The paper presents an FEM analysis of the collapse behaviour of thick tubes (10

Lagrangian Discrete Parcel Simulation Of River Ice Dynamics

Abstract: A coupled two-dimensional model for river ice dynamics is developed for simulating ice movement and jamming in river reaches with complex geometries. For the hydrodynamic simulation an Eulerian finite element method is used. A Lagrangian discrete parcel scheme, based on the smoothed particle hydrodynamics method, is used to simulate the movement of surface ice. The model is applied to the Grass Island Pool area of the upper Niagara River.

Wave-Current Forces On Vertical Piles In Side-By-Side Arrangement

Abstract: The in-line and lift forces on vertical piles in a side-by-side arrangement induced by both irregular waves and currents were investigated experimentally. The characteristics in both time and frequency domain of in-line, lift and resultant forces as well were analyzed. The grouping effect coefficients of in-line and resultant forces on the piles related to a KC number and relative spacing parameters are given. Also, a comparison is made of the magnitude and direction of resultant forces on vertical piles in a side-by-side arrangement with the corresponding values for a single cylinder. The range of KC number tested is 6~60; the range of Reynolds number, (O.55~3.43)*104.

Effects Of Wave Directionality On The In-Line Loading Of A Vertical Cylinder

Abstract: This paper presents results from an experimental investigation into the loading on a rigid vertical circular cylinder in irregular unidirectional and multidirectional waves in water of uniform depth. The ambient flow at the location of the cylinder was measured directly using perforated-ball velocity meters (PVMs), in order to avoid uncertainties associated with the use of wave theories. Reynolds numbers were in the range of 104 to 7 X 104, and Keulegan-Carpenter numbers ranged from o to 16. In-line Morison coefficients and root-mean-square force coefficients are estimated from the loading measured on force sleeves at three elevations along the length of the cylinder. Results are presented for the probability distributions of forces and peak forces, and for Morison and rms force coefficients. Effects of wave directionality are compared with Dean's analytical predictions.

Impulse Turbine With Self-Pitch-Controlled Tandem Guide Vanes For Wave Power Conversion

Abstract: Experimental investigations directed towards improving the performance of the impulse turbine with self-pitch-controlled guide vanes are reported. Two types of tandem guide vanes are presented and tested: those with variable-pitch flat-plate and those with variable-pitch splitter. The results show that an impulse turbine of higher efficiency can be developed through the use of the splitter. It is found that, in this case, the upstream guide vanes act as a slotted nozzle blade row and the downstream vanes as a tandem diffuser blade row by changing the pitch angle of the splitter automatically in a reciprocating flow. It is also found that the running and starting characteristics of this turbine in the reciprocating flow can be evaluated from the performance of the turbine with fixed nozzle and diffuser vanes in a unidirectional steady flow.

A Re-Evaluation Of Cobalt-Rich Crust Abundance On The Pacific Seamounts

Abstract: Cobalt-rich crusts on seamounts have been considered a potential mineral resource for the 21st century, and mining programs have been assessed in Japan during recent years. The crust distribution characteristics are not yet clearly known, although the United States, Germany, France and Japan have been conducting its exploration. So far the potential reserve of the crusts is estimated on the basis of their exposed surface coverage on the sea floor. During the research vessel Hakureimaru No.2 Survey Cruise in 1991, gravity corings of cobalt-rich manganese crusts were tried at three seamounts near Marcus Island in the Pacific Ocean. The results show the existence of a significant amount of buried cobalt-rich manganese crusts beneath calcareous sediments at the depth of 1,500 to 2,500 m. Based on these results and the associated data, a preliminary re-evaluation of cobalt-rich crust abundance on seamounts was carried out. The re-evaluation concludes that there is a drastic change in the amount of the crust reserve.

Laboratory measurements of breaking inception and post- breaking dynamics of steep short crested waves

Abstract: Monochromatic short crested waves have been produced in a laboratory flume using a shaped wavemaker. Breaking has been observed to occur near the peak in the amplitude modulation of these waves, over a range of local wave steepness. The conditions for inception and the post-breaking morphology (breaker area, height, length) and dynamics were observed photographically, and are quantified. Breaking inception was observed over a wide range of wave steepness (Hi/gT2) from 0.011 to 0.0335. The latter is in excess of the Stokes limiting steepness for planar waves, and an explanation is suggested based on calculated reductions in crest orbital velocity due to short crestedness. Variations in breaker severity from one wave cycle to the next are observed and are believed to be due to the disturbances left in the water by preceding breakers. A small jet was observed at the initiation of the breaking wave, and a simple estimate of jet dimension is given, based on observed breaker growth rates.

Reliability analysis of production ships

Abstract: Reliability analysis of bottom panels of production ships is performed. Probabilistic models are used for still water bending moment (SWBM), vertical wave bending moment (VWBM) and dynamic pressure. A load combination factor is introduced for the combination of SWBM and VWBM based on observed ship operational data and stochastic process theory. The column approach is applied for design of longitudinally stiffened panels simultaneously subjected to axial compressive stress arising from the global bending moments and local bending stress arising from the lateral pressure. Probabilistic model uncertainty for the interaction model is included. The hogging condition is considered. Implied safety levels in DnV Rules are estimated by considering various design cases, which are found to be unconservative when the design VWBM is dominant. A more rational design format is then proposed. The corresponding load factors are calibrated based on rationally selected target safety levels.

A model for fatigue life prediction of offshore welded stiffened steel tubular joints using fm approach

Abstract: Welded steel tubular structures commonly used for offshore platforms are susceptible to fatigue failure due to stress concentration at welded joints and environmental loading which is cyclic in nature. Evaluation of the structural integrity of these structures in service requires information on fatigue crack growth modelling besides other data. Internally ring stiffened steel tubular joints are found to be efficient in reducing stress concentration and improving ultimate and fatigue strengths. Fatigue tests were conducted on seven internally ring stiffened T and Y joints under constant amplitude loading in air to study their fatigue behaviour. This paper presents the crack growth data obtained from the fatigue tests. It also describes a new model for fatigue crack growth prediction in internally ring stiffened tubular joints using fracture mechanics.

Laboratory Experiments Of Oil Spreading In Brash Ice

Abstract: Experiments were conducted in an ice basin to examine the behaviour and spreading rates of oil in brash ice. Sea water was used to grow ice sheets which were broken to produce the required ice cover. Two types of North Sea crude oils (Gullfaks and Oseberg) as well as Bunker fuel were used. Oil samples were spilled on the broken ice/slush covers at an approximately constant flow rate. Analysis of the video records gives maximum spill dimensions, spreading rates, and ratio of the oil-covered area to the total area of the spill. The results show that oil always flowed over the brash ice, and that surface tension plays a more significant role than viscosity in decreasing oil spreading. Measured slick dimensions were used to develop empirical expressions for the balance of gravity, surface tension and viscous forces.

Positioning Of A Floating Otec Plant By Surface Intake Water

Abstract: Under normal OTEC operating conditions, a floating body could be kept in a global position, within a given radius, without the need for mooring lines or thrusters if the momentum flux from the surface seawater intake or the emuent discharge is adequately distributed around the hull. This problem is studied in the presence of steady external forces that act on the platform-pipe system in regular seas, and a positioning model is developed. The model is then applied to a sample OTEC plant to determine the technical feasibility of the solution in particular cases. The proposed positioning model can be used to determine the distribution and amount of momentum flux necessary to keep the plant in position.

A Model For The Motion And Bending Of An Ice Floe In Ocean Waves

Abstract: A new model to represent the wave-forced motion and flexure of a single elastic ice floe of constant thickness is reported. The model predicts that resonance, i.e., perfect transmission, occurs when the ratio of the ice wavelength to the floe diameter assumes certain values, analogous to the propagation of electromagnetic radiation through a homogeneous slab. Features in the reflexion and transmission coefficients, and strain fields are discussed. FORMULATION OF PROBLEM where We nondimensionalize, scaling lengths by L and time by ,j L / g , and derive the following nondimensional boundary value problem, assuming the velocity potentiaii/J is periodic in time with radian frequency .Ja : Our approach is based on Tayler (1986). A solitary ice floe of width L is modelled as an elastic raft floating on the equilibrium surface (z = 0) of the ocean, which is assumed to be infinitely deep. We consider the two-dimensional motion and flexure of the raft when acted upon by a train of small amplitude surface waves propagating in the direction of the positive x-axis. The raft occupies the region °~x ~ L, z = 0. Over the raft we use the Bernoulli-Euler model for the deflexion, neglecting gravity effects:

Ultimate Behavior Of Multiplanar Double K-Joints Of Circular Hollow Section Members

Abstract: The results of a test on multiplanar double K-joints with circular hollow sections under balanced axial brace loading are reported. The objective of the test is to investigate influences of the brace diameter to chord diameter ratio /3 = diD, the transverse gap to chord diameter ratio 'I = glD, the longitudinal gap to chord thickness ratio ,. = glT and the out-of-plane angle ¢. on the static strength of the joints. These variables are varied in the following range: 0.22:0;,&;0.47, 0.04:0;'/:0;0.53, 2:0;'1:0;14 and ¢. = 60° or 90°. The influences of ,. and 'Ion the ultimate capacity of double K-joints are independent of each other. The ultimate capacity increases with the decrease of ,., but this increase is smaller than for K-joints. The influence of 'I depends on the failure type: for a constant value of /3,the ultimate capacity increases with 'I for the failure type without local deflections of the chord wall between the compression braces and decreases for the failure type with significant local deflections of the chord wall between the compression braces. The ultimate capacity of double K-joints is compared with previous research on double K and T-joints, as well as with AWS, IIW and Cidect design recommendations. Formulae for the ultimate capacity of double K-joints are proposed based on past and present results. INTRODUCTION NOMENCLATURE Tubular structures are widely used for offshore oil drilling and production. Mostly these are trussed space frames extending from the sea bottom to just above the sea level. In such space frames multiplanar joints are unavoidable. However, since studies in the ultimate behavior of multiplanar tubular joints have started only Received December 12, 1991: revised manuscript received by the editors October 23, 1992. The original version (prior to the final revised manuscript) was presented at the Second International Offshore and Polar Engineering Conference (ISOPE-92), San Francisco, USA, June 14-19,1992.

An Experimental Investigation Of The Mean And Dynamic Tensions In Towed Strumming Cables

Abstract: The mean hydrodynamic drag is shown analytically to playa critical role in the deployment of small-diameter lightweight cables. To examine this parameter, cable tensions accompanied by strumming were measured in a towing tank. Mean hydrodynamic tensions were found in excellent agreement with predictions based on Taylor's analogy. Augmentation of tangential drag coefficients due to strumming of up to 50% were measured, in excellent agreement with similar heat transfer measurements under lateral excitation. Large fluctuations in tension were measured and found to be due to strumming. Excellent theoretical and experimental agreement is reported. These fluctuations increase with cable length and may be important for the integrity and stability of long cables. Strumming was observed over the entire speed range. In the speed range of transition between the lower and next higher natural frequency, simultaneous excitation of significant amplitudes was observed for both modes in their spectral and times series records.

Unsteady Flow About Bluff Cylinders

Abstract: A general method is presented for solving the Navier-Stokes equations in two dimensions using the Random Vortex Method (RVM), which is particularly attractive for modeling highly separated flows New techniques are developed for the RVM to handle unsteady flows about arbitrarily shaped cylinders, which are allowed to undergo any prescribed motion Computations of the convective interaction of vortex blobs are achieved by a fast order N algorithm New expressions for such computational methods to calculate the forces on moving bodies are also given Various problems of engineering significance are studied by this methodology, including the unsteady accelerating motion of a cylinder, an oscillating current about a fixed cylinder and about a transversely-oscillating cylinder Numerical results are found to agree well with experimental ones and certain existing computations

Wave Field In A Laboratory Wave Basin With Partially Reflecting Boundaries

Abstract: This paper describes a numerical method for predicting the wave field produced by one or more segmented wave generators in a laboratory wave basin which may contain fully or partially reflecting walls or structures. The approach used is based on linear diffraction theory and utilizes a point source representation of the generator segments and any reflecting boundaries that are present. The method involves a partial reflection boundary condition which is discussed. Numerical results are presented for the propagating wave field in a rectangular basin containing two segmented wave generators whose motions correspond to specified wave parameters applied to the snake principle. Cases which are considered fully absorbing and partially reflecting beaches along the basin sides, and a fully reflecting circular cylinder located in the test area of the basin. The method appears able to account adequately for the effects of wave diffraction and partial refections, and to predict the generated wave field realistically.

Experimental validation of directional wave maker theory with side wall reflections

Abstract: Dalrymple recently developed a directional wave maker theory which is capable of simulating a desired oblique flow planar wave train at any predetermined cross-section in a wave basin. This theory, which uses intentional refections from side walls, can be used to significantly enlarge the testing area in the basin over which the sea state is expected to be homogeneous. A numerical validation of this theory was recently undertaken, and the findings were published in Mansard et al. (1992). This paper reports of the results of an extensive series of experimental investigations undertaken to further validate this theory for the case of a basin with a sloping bottom.